The rates of photosynthesis and respiration define the role of lakes and oceans in the global carbon cycle: if photosynthesis exceeds respiration then the water is a sink for CO2, while if respiration exceeds photosynthesis then it is a source of CO2. Due to the relative ease of incubation-based 14C primary production methods, the rate of photosynthesis has been measured many hundreds of thousands of times. Oceanographers and limnologists achieved a global view of photosynthesis rates by the late 1960s and the global database of 14C primary production data continues to grow. This information has provided the biological basis for potent satellite- and model-based analyses of ocean biogeochemistry. In contrast, the rate of respiration has only been measured about 2000 times and, thus, there is no global-level understanding of aquatic respiration rates; this is a profound gap in our knowledge. In the ocean, the paucity of respiration rate data has fueled an intense debate over whether the ocean is a net source or sink of CO2 on a global scale. Furthermore, there is very little incubation-based respiration data to assess regional scale CO2 balances, even within the coastal waters of the United States. Given the importance of understanding CO2 balance from both scientific and geopolitical standpoints, the demand for respiration rate data will skyrocket in the next few years.
One of the primary reasons that the study of aquatic respiration has fallen so far behind the study of photosynthesis is that respiration rates are difficult to measure. Heterotrophic bacteria play an important role in attenuating the flux of particulate organic carbon (POC) flux through the twilight zone (i.e. shallow but dark waters); along with zooplankton, heterotrophic bacteria contribute to both particle disaggregation and attendant organic carbon respiration. The rates of heterotrophic bacterial processes in the twilight zone are substantially slower than in the euphotic zone, and accurately measuring these rates in the twilight zone presents numerous technical challenges. Many current technologies for measuring photosynthesis or respiration involve removing samples from their environment for incubation. A widely-applied tactic is to bring twilight zone water samples to the surface and apply scaled up versions of incubation-based methods originally designed for use in the euphotic zone; these methods are applied at atmospheric pressure and the effects of depressurization are assumed to be negligible. This has been done for measuring rates of tritiated thymidine incorporation by heterotrophic bacteria. The thymidine incorporation rates can be compared to rates of bacterial carbon demand (BCD) and, although this rate conversion imparts large uncertainties, comparisons can be made between BCD, zooplankton carbon demand, and the loss of sinking POC flux. These results showed, both in North Pacific subtropical gyre and the subarctic North Pacific, that twilight zone BCD greatly exceeded the loss of POC, suggesting either: 1) that BCD was overestimated; 2) that POC flux attenuation was underestimated; or 3) that there were additional, large sources of organic carbon to the twilight zone. Therefore, the rates of BCD should be better constrained. For example, enzymatic POC hydrolysis rates determined during testing can add some additional bounds to the BCD dataset. However, further bounds are still needed.
Another surface method is to incubate seawater (light and dark) and measure the decrease in oxygen concentrations using the Winkler titration method, as is known in the art. The Winkler titration can be messy, time consuming, and error prone and, as such, a major impediment to the study of respiration at sea. Alternative methods have been attempted, such as measuring oxygen with electrodes or tracking an increase of CO2, but these have not shown the reliability and sensitivity of the Winkler titration. Unlike photosynthesis, respiration is not confined to the sunlit waters of the ocean's surface. This is because organic particles from photosynthetic organisms sink from the surface waters to the mesopelagic, where most of them are ultimately respired. Since the Winkler titration is a wet chemical method, oxygen measurements of mesopelagic waters must be conducted on the deck of ship. This again introduces biases caused by depressurization of the microbes responsible for respiration. Some studies that have addressed the impact of pressure on microbial respiration suggest that the depressurization biases may be quite large, even at relatively shallow depths (e.g., 100s of meters).
Radioisotope methods can provide simple yet sensitive measurements; however, these methods are not generally available for measuring respiration rates. One method is performed partially in situ. This approach requires that the device sample and filter the incubation, and that these filters be retrieved using a research vessel in order to quantify the desired process (e.g., to measure photosynthesis and respiration). However, this method (and other radioisostope systems) have the disadvantage of being subject to regulatory constraints associated with the use of radioactive substances in the ocean.
Photosynthesis and respiration are arguably the defining parameters of carbon cycling in aquatic ecosystems (freshwater and saltwater), and are also primary components of oxygen demand measurements (i.e., biological oxygen demand (“BOD”)) in the water quality community. Almost every municipality makes BOD measurements as part of their wastewater management operations. The ability to easily make such measurements is also of interest in the oceanographic community and plays a key role in the Ocean Observatories Initiative.
Accordingly, there exists a need in the art for a reliable, cost-effective, in situ system for analyzing aquatic parameters of interest, such as determining respiration rates.